The invention relates to a method for determining strength of co-channel signals, the method being used in a digital radio system comprising several base stations and subscriber terminals, which operate as transmitters and receivers for signals that are desired signals and/or interference signals and that propagate in channels and that comprise at least one predetermined sequence characteristic of the transmitter, the signals being sampled when they are received.
The invention also relates to a method for determining strength of co-channel signals, the method being used in a digital radio system comprising several base stations and subscriber terminals, which operate as transmitters and receivers for signals that are desired signals and/or interference signals and that propagate in channels and that comprise at least one predetermined sequence characteristic of the transmitter, the receivers comprising diversity branches.
The invention further relates to a receiver arranged to be used in a digital radio system comprising several base stations and subscriber terminals, which operate as transmitters and receivers for signals that propagate via channels and that are desired signals and/or interference signals and that comprise at least one predetermined sequence, the receivers being arranged to sample the signals.
In a cellular radio system, the quality of a connection between a base station and a subscriber terminal varies continuously. This variation is due to the attenuation of the radio waves as a function of distance and time on a fading channel, and to the interfering factors occurring over the radio path, such as other signals on the same channel. In a cellular radio system, it is necessary to use the same frequency repeatedly in different cells due to the limited frequency range. In such a case, a receiver operating on a frequency usually receives co-channel signals that originate from several transmitters and that have propagated along several different paths and are therefore multipath signals. The capacity of the receiver can be improved by using common detection of the strongest interference. For this purpose, the receiver should determine the strength levels of different signals and perform co-channel estimation on the signals. However, the problem in the prior art is locating the strongest signals.
The invention can be applied particularly in a digital cellular radio system where transmission occurs on a time division modulation basis. In such a case, one time slot typically comprises one burst transmitted by a subscriber terminal or a base station. A burst sequence usually comprises a start, data, training and stop sequence. The training sequence is predetermined. The length of the start and stop sequences is only a few symbols, but the data and training sequences usually have a length of several dozens of symbols. The training sequence represents a predetermined sequence. Since the number of known symbols in the training sequence is relatively small, whereas the number of signals is often rather great, the problem in the prior art is that the training sequence is not useful for the formation of signal strength estimates and the strongest signals and/or interference cannot therefore be located.
Multipath reception often utilizes diversity receivers. The most common diversity receivers combine the signals before or after the detection and they include for example selective combining, maximal-ratio combining and equal-gain combining. In prior art combining, strong interference is easily combined with the actual signal since the combining is based on the signal-to-noise ratio. A diversity branch that does not receive a signal or receives a wrong signal should not be used in the combining.
The purpose of the present invention is to facilitate the finding of both desired signals and interference and to make diversity combining more effective.
This is achieved with a method of the type described in the preamble, characterized by forming subsets of predetermined sequences from the predetermined sequences related to the signals to be received, performing channel estimation separately for each subset based on the samples of the received co-channel signals and the subsets formed, and a channel estimate related to each subset being formed as a result of the estimation, and forming by means of the channel estimate, an estimate signal for the subset, the signal being compared to the received signal, and forming, by means of said comparison, a strength estimate that describes the strength of at least one signal of the subset and that is used to select at least one subset with the greatest signal strength.
The method according to the invention is also characterized by forming subsets of predetermined sequences from the predetermined sequences related to the signals to be received, performing channel estimation separately for each subset based on the samples of the received co-channel signals and the subsets formed, and a channel estimate related to each subset being formed as a result of the estimation, and forming, by means of the channel estimate, an estimate signal for the subset, the estimate being compared to the received signal, and forming, by means of said comparison, a strength estimate that describes the strength of at least one signal of the subset and that is used when at least one subset with the greatest signal strength is selected, and forming from the channel estimates a correlation result between the diversity branches, the result being used to decrease the amount of interference, for example.
The receiver according to the invention is characterized in that the receiver comprises sequential means and grouping means for forming subsets of predetermined sequences from the predetermined sequences that are related to the signals to be received and that are stored in the sequential means, and the receiver comprises channel estimation means for carrying out channel estimation separately for each subset, the estimation being arranged to take place by means of the received signal samples and the subsets formed, and to form a channel estimate related to each subset, and the channel estimation means are arranged to form an estimate signal by means of the channel estimate and the predetermined sequence, and to compare the received signal and the estimate signal for the subset, and the channel estimation means are arranged to form, by means of said comparison, strength estimates for the subset signals.
The method according to the invention provides considerable advantages. The invention enables the finding of the strongest signals, especially interference signals, with great likelihood. Another advantage is particularly that without restricting the number of the co-channel signals it is possible to form the signal strength estimates by using a short predetermined training sequence. For example in a GSM system, only a few dozen predetermined symbols can be used in a training sequence, but in the present method the number of the required parameters to be estimated is smaller. In diversity combining, only the desired signals are combined and the interference is suppressed by means of the method according to the invention, so that the signal-to-noise ratio is improved and the symbol detection is more reliable. The arrangement according to the invention can be applied especially in interference elimination.